RichTek RT34063A (DC-DC converter) - use in Pb-acid battery charger circuit

[6SN7WGTB]

Just dismantled a PSU which is marked "12V 900mA DC", but I know was to charge a lawnmower starter 12V battery.

It outputs about 14.3V DC, with about 50mV ripple off-load.

The circuit is essentially an iron-core standard transformer, bridge rectifier, smoothing, then the 8-pin DIL chip driving an output transistor, which goes via an inductor to the output.

The design is close to that shown on various datasheets for the chip - example here https://www.ventronchip.com/pdf-f5/RT34063AGS.pdf

What I'm particularly interested in is why a basically linear design uses this chip - is it specifically as it is used to charge a Pb-acid battery?

Does the voltage output somehow regulate down as the battery reaches the more nominal 13.2V?

I'm assuming the external tr is for current boost?

[Benta]

It's just a voltage regulator like a 7812, in this case a switching one.
14.3 V with added current limiting sounds about right for a lead-acid battery.
Never heard of "Richtek" in my life. The original part is Motorola/onsemi MC34063, and you'll find lots of application notes on the web.
Pretty simple design, I'd say.
The transformer part is to prevent you from killing yourself.

[6SN7WGTB]

Good spot re the MC/Ti basis - don't know why my search didn't spot that.

Looking at the application note, I think the topology conforms to a boost design.

So, is all this complexity which introduces 20-40kHz fuzz into a linear circuit done simply to get some boost out of a more 'standard' and/or smaller transformer hen an even simpler LM317 would give the required output?

Just seems odd when an SMPS design ground up would have cut the box down by 75%

[mikerj]

Never heard of "Richtek" in my life.

That is quite surprising as they've been around for at least 20 years.  They specialise in power solutions like regulators and DDR memory termination.

[TheMG]

So, is all this complexity which introduces 20-40kHz fuzz into a linear circuit done simply to get some boost out of a more 'standard' and/or smaller transformer hen an even simpler LM317 would give the required output?

Just seems odd when an SMPS design ground up would have cut the box down by 75%

It's not to get "boost" out of the transformer. They likely wanted some voltage regulation (since AC mains voltage fluctuates) as well as current limiting not to burn up transformer when charging a heavily discharged battery.

Sure, a linear regulator would also work for this purpose but at lower efficiency and a lot of added heat dissipation.

As for why they didn't use a full SMPS design to start off with... who knows.

[mag_therm]

If it is for outdoor use on garden tool which likely has  accessible 12V terminals, what would be alternative to mains frequency isolation transformer?

[Phoenix]

Sounds like a continually reused legacy design.

what would be alternative to mains frequency isolation transformer?

A small flyback.